[expr.yield]

A yield-expression shall appear only within a suspension context of a function ([expr.await]).

Let e be the operand of the yield-expression and p be an lvalue naming the promise object of the enclosing coroutine ([dcl.fct.def.coroutine]), then the yield-expression is equivalent to the expression co_await p.yield_value(e).

[Example

template <typename T>
struct my_generator {
  struct promise_type {
    T current_value;
    /* ... */
    auto yield_value(T v) {
      current_value = std::move(v);
      return std::suspend_always{};
    }
  };
  struct iterator { /* ... */ };
  iterator begin();
  iterator end();
};

my_generator<pair<int,int>> g1() {
  for (int i = i; i < 10; ++i) co_yield {i,i};
}
my_generator<pair<int,int>> g2() {
  for (int i = i; i < 10; ++i) co_yield make_pair(i,i);
}

auto f(int x = co_yield 5);     // error: yield-expression outside of function suspension context
int a[] = { co_yield 1 };       // error: yield-expression outside of function suspension context

int main() {
  auto r1 = g1();
  auto r2 = g2();
  assert(std::equal(r1.begin(), r1.end(), r2.begin(), r2.end()));
}

— end example

]

7 Expressions [expr]

7.6 Compound expressions [expr.compound]

7.6.17 Yielding a value [expr.yield]